1. Field of the Invention
The invention concerns photopolymerizable pressure-sensitive adhesive tapes comprising a plurality of contiguous layers which cannot be delaminated. Each of the layers comprises a photopolymerized matrix of polymeric chains, and at least one of the outer layers is photopolymerized to a pressure-sensitive adhesive state. This invention also concerns a process for concurrently coating tapes incorporating such layers.
2. Description of the Related Art
The invention concerns photopolymerizable pressure-sensitive adhesive tapes. U.S. Pat. RE No. 24,906 (Ulrich), reissued on Dec. 20, 1960, discloses pressure-sensitive adhesive tapes, the adhesive layers of which comprise copolymers consisting essentially of monomers of acrylic acid esters of non-tertiary alkyl alcohols having from 1-14 carbon atoms, and at least one monomer copolymerizable therewith.
U.S. Pat. No. 4,181,752 (Martens et al.) discloses a process for making pressure-sensitive adhesive tape which involves the photopolymerization of the alkyl esters of acrylic acid and the modifying monomers to form the acrylate copolymers. It is also disclosed that the intensity and spectral distribution of the irradiation must be controlled in order to attain desirably high peel resistance and cohesive strength. The process disclosed is preferably carried out in the absence of oxygen and air which inhibit the polymerization reaction. Thus, it is normally carried out in an inert atmosphere such as nitrogen, carbon dioxide, helium, argon, etc. Air can also be excluded by sandwiching the liquid photopolymerizable mixture between layers of solid sheet material and irradiating through the sheet material. Each layer must be coated and cured before the addition of another layer.
U.S. Pat. No. 4,243,500 (Glennon) discloses a pressure-sensitive adhesive formed from a composition comprising mono-functional unsaturated acrylate ester monomer, essentially saturated tackifying resin polymer dissolved in the acrylate ester, non-crystallizing elastomeric material also dissolved in the acrylate ester and an initiator responsive to ultraviolet light or other penetrating radiation such as electron beam, gamma, or X-ray radiation. The intensity of the lamps taught by Glennon is much greater than those taught by Martens.
One embodiment of a pressure-sensitive adhesive tape is commonly called a "transfer tape" in that it typically has a low-adhesion liner from which it is transferred when used. Such a tape can also be linerless as disclosed in U.S. Pat. Nos. 2,889,038 (Kalleberg) and 4,522,870 (Esmay). One embodiment of the invention, like those tapes of U.S. Pat. Nos. 4,223,067 (Levens), and 4,514,615 (Esmay), has a foam-like appearance and character, even though it is not a foam.
The double-coated pressure-sensitive adhesive tape of U.S. Pat. No. 2,889,038 (Kalleberg) comprises a flexible support having on opposite faces chemically different pressure-sensitive adhesive layers which are physically incompatible, thus enabling the tape to be wound directly upon itself into a roll for storage and shipment. The tape is made by successively coating and drying solutions of two different pressure-sensitive adhesives onto opposite faces of a flexible web. To test for the incompatibility of the two pressure-sensitive adhesives, a solution of one of the pressure-sensitive adhesives is coated onto an undried coating of the other, and the coatings are simultaneously dried at room temperature for 24 hours to evaporate the solvents. Physical incompatibility is demonstrated by peeling the dried layers apart.
The double-coated pressure-sensitive adhesive tape of the above-cited Esmay patent is similar to that of the Kalleberg patent except that both adhesive faces can have truly high performance, and the adhesive layers at the two faces of the flexible web do not need to be either chemically different or physically incompatible. This is achieved when the pressure-sensitive adhesive at each of the faces is a polymer of predominantly alkyl acrylate, substantially solvent-free, and crosslinked. The Esmay patent states: "It is surmised that if the adhesive were not substantially solvent-free, the solvent would allow the polymer chains to knit across adjacent convolutions during prolonged storage in roll form, such that perfect separation could no longer be assured. In the present state of the art, it would not be commercially feasible to coat a pressure-sensitive adhesive from solution and obtain a pressure-sensitive adhesive layer which is substantially solvent-free. To keep the amount of solvent to a minimum, the (Esmay) tape is preferably made using photopolymerization as in U.S. Pat. No. 4,181,752 (Martens et al.)" (col. 2, lines 21-32).
The Esmay patent discloses that a "technique for enhancing immediate adhesion to relatively rough or uneven surfaces is to incorporate glass microbubbles into the pressure-sensitive adhesive as taught in U.S. Pat. No. 4,223,067 (Levens)" (col. 4, lines 31,35). Because the microbubble-containing tape of the Levens patent has a foam-like appearance and character, it is sometimes called a "foam-like" tape even though its pressure-sensitive adhesive layer is substantially free of voids except for the hollow spaces within the microbubbles. The Levens patent in turn teaches that where it is desired to adhere the foam-like tape "to a surface to which its pressure-sensitive adhesive layer would not form a strong bond, it may be desirable to apply to one or both of its faces of its microbubble-filled adhesive layer a layer of unfilled pressure-sensitive adhesive which is especially selected for adhesion to that surface" (col. 4, lines 9-15). Such microbubble-free surface layers can also provide substantially increased cohesive strength, especially at high temperatures. Multiple microbubble-free surface layers can have different adhesive properties, each selected for good adhesion to a certain surface. Because the application of those added layers substantially increase the cost of the foam-like tape, less expensive foam-backed tapes have dominated the market for uses requiring immediate adhesion to rough or uneven surfaces.
The microbubbles can be glass as in the examples of the Levens patent, or they can be polymeric as described in U.S. Pat. No. 3,615,472 (Morehouse et al.) or U.S. Pat. No. 4,287,308 (Nakayama et al.).
U.S. Pat. Nos. 4,710,536 and 4,749,590, (Klingen, et al.), disclose the use of certain hydrophobic silicas as fillers for photopolymerized pressure-sensitive adhesive tapes. Preferred silicas have surface areas of at least 10 m.sup.2 /g. It is disclosed that the presence of the filler increases the internal strength of the tape.